1. Field of the Invention
This invention is directed to a stable, more useful form of lithium, lithium alloys and substrates coated with lithium, particularly suited for use as anodes in secondary electrochemical cells. This invention also relates to high energy density electrochemical cells. A lithium anode in such a cell is highly desirable because the use of a lithium anode results in useful voltage at a very low equivalent weight. The anode is the electrode which undergoes oxidation during the discharge portion of the discharge-charge cycle.
Lithium, however, tends to react with the organic electrolyte in the cell. When this happens, the reacted lithium is lost for recycling purposes thereby reducing the efficiency of the cell. Such loss can lead to premature destruction of the cell. The reaction products of the lithium and the organic electrolyte are deposited on the anode, eventually effectively insulating the anode from participation in the electrochemical reactions of the cell, thereby leading to cell failure.
Furthermore, lithium tends to form dendrites during recharge. These dendrites, which are nodular, poorly adhering forms of lithium, tend to fall from the anode and become isolated from the electrochemical reactions of the cell, a process known as "lithium isolation". Additionally, the dendrites may eventually lead to short circuiting of the cell if bridging by the lithium from the anode to the cathode takes place.
This invention is directed to enhancing a surface of lithium metal by creating a conducting layer of polymeric design. The enhanced lithium surface finds use in the fabrication of improved electrochemical cells and batteries made from lithium electrodes.
2. Background of the Invention
Electrochemical cells containing an anode, a cathode and a solid, solvent-containing electrolyte are known in the art and are usually referred to as "solid batteries". The use of certain of these solid batteries over repeated charge/discharge cycles is substantially impaired if the battery exhibits a drop in charge and discharge capacity over repeated cycles as compared to its initial charge and discharge capacity.
Specifically, solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix as well as a suitable inorganic ion salt. The inorganic matrix may be non-polymeric [e.g., .beta.-alumina, silicon dioxide lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, polyethylene oxide, polypropylene oxide, polyethylenimine, polyepichlorohydrin, polyethylene succinate, and an acryloyl-derivatized polyalkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrochemical cells containing a solid electrolyte a polymeric matrix may suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain a solvent (plasticizer) which is typically added to the matrix in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrochemical cell.
To make a solid electrolyte, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid electrochemical cell.
Notwithstanding the above, the initial capacity of solid batteries is often less than desirable. Moreover, even when the initial capacity of the solid battery is relatively high, such solid batteries often exhibit rapid decline in capacity over their cycle life.
Specifically, the cumulative capacity of a solid battery is the summation of the capacity of a solid battery over each cycle (charge and discharge) in a specified cycle life. Solid batteries having a high initial capacity but which rapidly lose capacity over the cycle life will have low cumulative capacity which interferes with the effectiveness of these batteries for repeated use.
The normal passivation layer of lithium surfaces in contact with the described electrolytes are relatively brittle and unstable. When the battery is cycled, the lithium is stripped from the anode on the discharge half-cycle and is plated on the recharge half-cycle. The weak passivation layer is disrupted by the cyclic process and the lithium metal reacts further with the electrolyte. Consequently, the lithium is stripped and plated selectively over the anode surface, causing the growth of dendrites or nodules which can eventually short the cell through the electrolyte.
U.S. Pat. Nos. 5,147,739 and 5,110,696 suggest the use of composite anodes comprising lithium or lithium alloy substrate in combination with one or more lithium insertion compounds. The intercalation compound may be adhered, mixed, embedded, or otherwise contacted as a finely dispersed layer, coating, laminate, or mixture with the lithium substrate. Such anodes, like the use of lithium/carbon anodes (U.S. Pat. No. 5,028,500), and the use of lithium alloys, although less reactive towards the electrolyte, provide lower energy density.
It would be advantageous if the lithium surface could be enhanced by a strong and stable layer which is ionically and electronically conducting.
In view of the above, the art is searching for methods to enhance the cumulative capacity of such solid batteries. It goes without saying that increases in the cumulative capacity of solid batteries would greatly facilitate their widespread commercial use.